The present invention relates to a channel selector in a receiver such as in radio and television receivers, and more particularly to a PLL (Phase Locked Loop) frequency synthesizer type channel selector that is available in television receiver.
In a PLL frequency synthesizer type channel selector, an oscillation frequency of a local oscillator is controlled according to the PLL technique. More particularly, an oscillation output from a local oscillator is applied to a programmable frequency-divider and is divided in frequency according to frequency-division ratio data which are set in the programmable frequency-divider. The frequency-divided output is applied to one input of a phase detector. The phase detector detects a phase difference between the frequency-divided output and an oscillation output from a reference oscillator which is applied to the other input of the phase detector. The detector output is fed to a low-pass filter, and the output of the low-pass filter is applied to the local oscillator as a control signal for controlling the oscillator frequency of the local oscillator. Owing to this phase synchronizing loop, the oscillation frequency of the local oscillator can be controlled so that the phases of the respective inputs to the phase detector may coincide with each other. Accordingly, a desired local oscillation frequency can be obtained by varying the frequency-division ratio data the programmable frequency-divider. This local oscillation frequency is mixed with the received frequency to derive an intermediate frequency signal, and a desired broadcasting channel can be received by demodulating the derived intermediate frequency signal. Accordingly, a receiver can be set so as to receive a transmission frequency from a desired broadcasting station by properly selecting the frequency-division ratio data. Therefore, in this type of channel selector, the channel selection operation is easy, and it is possible to realize stable operation free from deviation in synchronization.
On the other hand, however, in the event that the transmission frequency of the broadcasting station deviates from the regular frequency allotted to that station, in the above type of channel selector it is impossible to achieve optimum tuning because the local oscillation frequency is derived according to preset frequency-division ratio data. In other words, since the frequency-division data are preset and stored in a channel selector so that the local oscillation frequency corresponding to the regular frequency alloted to the desired broadcasting station may be derived, if the transmission frequency deviates from the regular frequency, then the intermediate frequency obtained by mixing the transmission frequency with the local oscillation frequency also deviates. As a result, optimum tuning becomes impossible.
Accordingly, in order to achieve optimum tuning, the local oscillation frequency must be controlled in dependence upon the transmission frequency. If the deviation of the transmission frequency from the regular frequency (hereinafter called "offset") is small, then the receiver can be automatically adjusted to the optimum tuning point by additionally providing an AFT (Automatic Fine Tuning) circuit. This AFT circuit acts to vary the frequency-division ratio data so that the received intensity of the transmission signal may be maximized, and to thereby compensate the offset, that is, the deviation of the received transmission signal from the regular frequency. In other words, if there exists any frequency deviation in the transmission frequency, then the intermediate frequency derived by mixing the received transmission frequency with the local oscillation would deviate from the regular intermediate frequency by a corresponding amount. Accordingly, if the derived intermediate frequency signal is detected by an FM-detector, the detector output would be at a voltage corresponding to the frequency deviation. Therefore, the local oscillation frequency is varied by varying the frequency-division ratio data so that the output voltage of the FM-detector may be reduced to zero. As a result, the local oscillation frequency is made to correspond to the transmission frequency having the frequency deviation, and thereby optimum tuning can be achieved.
Here it is to be noted that when the frequency deviation is large, even with the fine tuning operation achieved by the AFT circuit, the receiving frequency of the receiver cannot be tuned to the deviated transmission frequency. This is because the frequency range where fine tuning by means of the AFT circuit is possible, is determined by the so-called S-curve characteristics of an FM-detector. The frequency range where the detection relying upon the S-curve of an FM-detector is possible, is generally the range of .+-.1 MHz with respect to the center frequency. In other words, in the case where the transmission frequency from a broadcasting station has an offset of at most .+-.1 MHz, an FM-detector can generate a detector output corresponding to the magnitude of the offset, and hence the receiving frequency of the receiver can be tuned to the deviated transmission frequency through the AFT operation. However, among broadcasting stations there stations which transmit at a frequency having an offset of 1.3 MHz or more, especially in the case of cable TV broadcasting (hereinafter abbreviated as CATV) or like. Besides the CATV, VHF broadcasting which is transmitted through a cable also has a considerably large offset. When the transmission frequency has such a large offset, a detector output would not be obtained from the FM-detector, and as a result, tuning cannot be achieved through the fine tuning operation of the AFT circuit.
Therefore, as a provision for a transmission frequency having a large deviation from the regular frequency, the channel selector is equipped with a manual fine tuning function for varying the local oscillation frequency by externally varying the frequency-division ratio data in a forced manner. More particularly, the frequency-division ratio data can be forcibly varied by operating manual fine tuning keys provided in the channel selector. If the frequency-division ratio data are varied, then the local oscillation frequency is also varied by a corresponding amount. As a result, even in the event that a transmission frequency is received having such a large offset that fine tuning cannot be achieved by the AFT circuit, optimum tuning becomes possible.
However, the heretofore known channel selectors have had the following shortcoming. That is, assuming now that a TV viewer, for instance, has selected a particular channel, where the broadcasting station is transmitting at a frequency having a large offset, as is the case with CATV, then he can enjoy the TV program with the receiving condition optimized by operating the manual fine tuning keys in the above-described manner. Now let it be assumed that the TV viewer switched from the channel of that broadcasting station to another channel, and then again selected the previous channel. In response to the last channel selection, the frequency-division ratio data corresponding to the selected channel are set in the programmable frequency-divider, and hence a predetermined local oscillation frequency signal is generated. However, the transmission frequency of the re-selected channel has a large deviation from the regular frequency. Therefore, unless the manual fine tuning keys are again operated, the optimum receiving condition cannot be established. As described above, the channel selectors of the prior art were troublesome, in that when a channel for which manual fine tuning was once effected again selected, operation of the manual fine tuning keys had to be effected again. This troublesomeness was greatly enhanced when the channel selector was in the operation mode for receiving CATV.